There are three basic levels of control of aging: 1) the proximate causes leading to senesence in particular cell lines or tissues, 2) the regulatory processes that yield an integrated life history, and 3) the evolutionary forces that select upon life history traits to determine the aging patterns found in the species. We have been able to study the control of aging at all three levels with the abnormal abdomen (aa) syndrome in Drosophila mercatorum and with the insertion-induced bobbed (bb) syndrome in D. hydei. Both syndromes are caused by insertions that inactivate a large proportion of the 28S ribosomal genes found on the X chromosome. However, the aging effects are modulated by controlling somatic replication of inserted versus noninserted genes in D. mercatorum, whereas quantitative variation in the proportion of inserted genes appears to modulate the syndrome in D. hydei. Both syndromes are associated with a slow-down in larval developmental time, increased early adult fecundity, and decreased adult longevity. These syndromes are adaptive under natural environmental conditions that result in young adult age structures. The genetic architecture of aa will be examined, with the initial focus being on the impact of somatic replication and quantitative variation in insert proportion on the aging phenotypes. Autosomal modifiers will also be isolated and characterized, and it will determined if these modifiers can alter the pattern of pleiotropy. The control of aa expression in males as modulated by the Y chromosome will be investigated, with particular emphasis on the significance of variability in the Y-linked rDNA complex. The adaptive significance of aa expression in males will be determined by experimentation and field studies, including its impact on system of mating. We will determine the life history effects of quantitative variation in insert proportion for bb, both in the laboratory and in the field. Field studies will be used to test hypotheses about the adaptive significance of bb. Artificial selection experiments will also be used for this purpose, and parallel experiments will be done in D. mercatorum in order to test hypotheses about differential response to selection in the rDNA multigene families in these two species.